1. Field of the Invention
The present invention relates generally to wellhead assemblies, more specifically to assemblies and methods for connecting a horizontal tree to a wellhead housing.
2. Background of the Invention
Well fluid from a subsea well typically flows up a string of production tubing to a subsea wellhead. Sometimes well fluid is transmitted through a production riser to a Christmas tree on a vessel at the surface of the sea. It is often desirous however to transport the well fluid through a subsea Christmas tree to a collection facility or processing site. In either situation, the production riser or the subsea tree mounts to the wellhead housing. Typically, the production riser or subsea tree has a connector that receives an upper portion of the wellhead housing and then engages a grooved profile on the wellhead housing with a plurality of dogs.
Previously, the connector included a fluted receiving portion that receives and lands on the upper portion of the wellhead housing. The plurality of dogs align with the grooved profile on the outer surface of the wellhead housing when the connector lands on the wellhead housing. A piston housed with the connector slides axially up and down. The piston typically engages the dogs to cam the dogs, which engage the grooved profile of the wellhead housing. When the dogs are cammed radially inward, the connector and the dogs are in their locked positions. The dogs can be biased radially outward, or the dogs can be actuated radially outward upon upward movement of the piston in order to cam the dogs radially outward, or unlocked position.
The piston is actuated with hydraulic fluid that is injected into annular chambers formed around upper and lower portions of the piston with seals. When the seals function properly, hydraulic fluid is injected into an upper chamber to actuated the piston axially downward and thereby move the dogs into their locked position to engage the wellhead housing. Hydraulic fluid is injected into the lower chamber to actuate the piston axially upward to unlock the dogs from the wellhead housing.
Previously, when the seals failed so that hydraulic fluid could not build enough pressure to unlock the connector from the wellhead housing, a secondary piston assembly that was typically located external to the connector was used to lift the piston and unlock the connector. External piston assemblies are typically cumbersome and heavy, which causes the operator to take up valuable space around the subsea tree and wellhead assembly. Additionally, the parts of the connector that were engaged by the external piston had to be capable of sustaining the additional force of being pulled or pushed externally by the external piston.